1. Field of the Invention
The present invention relates to a semiconductor device having a circuit constituted by thin film transistors (referred to as TFTs) and method for manufacturing thereof. For example, the invention relates to an electronic device on which an electro-optical device typified by a liquid crystal display panel or a light emitting display device having an organic light emitting element is mounted as a component.
Further, in this specification, a semiconductor device means overall devices which can function by utilizing a semiconductor characteristic, and an electro-optical device, a semiconductor circuit, and an electronic device are all included in the semiconductor device.
2. Description of the Related Art
In recent years, technique for forming a thin film transistor (TFT) using a semiconductor thin film (having a thickness of approximately from several to several hundreds nm), which is formed over a substrate having an insulating surface, has attracted attention. A thin film transistor is applied broadly to electronic devices such as an IC or an electro-optical device, and has been rushed to be developed especially as a switching element of an image display device.
Especially, an active matrix liquid crystal display device provided with a switching element formed from a TFT in every display pixel placed in a matrix is actively developed.
In an active matrix liquid crystal display device, development of enlarging an effective screen area in a pixel region is promoted. To enlarge an effective screen area, an area occupied by a TFT (pixel TFT) placed in the pixel portion has to be made as small as possible. Furthermore, development of forming a driver circuit and a pixel portion over the same substrate is also promoted so as to reduce a manufacturing cost.
In a liquid crystal module mounted on a liquid crystal display device, a pixel portion which displays image, and a driver circuit for controlling the pixel portion, comprising a shift register circuit, a level shifter circuit, a buffer circuit, and a sampling circuit, each of which is based on a CMOS circuit, are formed over one substrate.
In the case of forming a driver circuit and a pixel portion over the same substrate, an area occupied by a region other than a pixel region, which is referred to as a frame portion, tends to be larger compared to that in the case of mounting a driver circuit by a TAB method. To reduce an area of the frame portion, the size of a circuit constituting a driver circuit also has to be reduced.
A pixel TFT is formed of an n-channel TFT and drives a liquid crystal by being applied voltage as a switching element. A pixel TFT of a liquid crystal display device is driven by an alternating current; therefore, a method referred to as frame inversion driving is often used. In this method, it is important for a characteristic required for a pixel TFT to sufficiently lower an OFF current value (a drain current flowing when a TFT is in OFF operation) in order to reduce power consumption.
Further, some techniques for making a line width of a gate electrode minute so as to reduce a transistor size are proposed.
For example, in Reference 1, technique of using a metal sidewall deposited on a side wall of a stepped portion as a gate is mentioned (Reference 1: Japanese Patent Laid-Open No. H4-212428).
In Reference 2, a thin film transistor (TFT) having a gate electrode having a sidewall shape is proposed (Reference 2: Japanese Patent Laid-Open No. 2003-282881).
The gate electrode having a sidewall shape mentioned in the above Reference 1 or Reference 2 is easy to have variation in its shape and thickness. Further, it is difficult to be electrically connected to the gate electrode having the sidewall shape of References 1 and 2 when employing a method for forming a normal contact hole, so it is required that the gate electrode is directly and electrically connected by overlapping a wiring formed from a conductive material different from that of the gate electrode. Therefore, contact resistance increases due to an influence of contact resistance between dissimilar metal. In addition, in the above Reference 1 or Reference 2, many steps (film formation step must be performed at least two times) are required to form a gate electrode and a gate wiring. In addition, a step for forming a step to form a sidewall is also required according to References 1 and 2.
Furthermore, in the TFT mentioned in Reference 2, a semiconductor layer is placed so as to straddle a stepped portion; therefore, it is difficult to obtain a semiconductor layer having uniform crystallinity in a crystallization step.
A channel width of a TFT is dependent on the width of a gate wiring in a conventional TFT, therefore, it is difficult to increase ON current of the TFT in case a channel length is long due to a wide gate wiring. Further, a channel length of a TFT cannot be shortened; therefore, gate capacitance is hard to be reduced and an integrated circuit including a TFT is prevented from operating at high speed.